Project Summary The goal of our research is to understand the cellular and molecular mechanisms behind the development of Stevens-Johnson syndrome. Adverse drug reactions (ADRs) are a serious drug safety concern that can be hard to predict and treat. Stevens-Johnson syndrome (SJS) is one of the most serious and life- threatening ADRs characterized by keratinocyte cell death and mucosal breakdown; patients often form lesions on their palms of hands and soles of their feet and blistering and necrosis of their eyelids, conjunctiva and cornea. The exact cellular and molecular mechanisms behind drugs triggering SJS is unknown. Using a Ca2+ imaging assay, we found certain members of the Mrgpr GPCR family are activated by multiple known SJS- causing drugs. To examine the immune effects of these SJS drugs, we developed a novel mouse treatment where daily oral ingestion of a SJS-causing drug resulted in WT mice forming mucosal secretion in their eyes and blister bleeding in their hindpaws, similar to symptoms seen in patients suffering from SJS. We generated a mouse line where the specific Mrgpr's open reading frame was deleted and replaced with GFP and this genetic removal prevented formation of the SJS-like phenotype. Strikingly, heterozygous Mrgpr +/GFP mice, mice retaining 1 functional copy of the certain Mrgpr allele, developed the SJS-like phenotype, implying an important role for this Mrgpr in drug-caused SJS. Further experiments revealed the Mrgpr was expressed in a subset of dendritic cells, cells known to play an important role in antigen-presentation and immune response initiation. In this proposal, we will use molecular, cellular, genetic approaches and a potential novel in vivo animal model to uncover the role of the Mrgpr-expressing dendritic cells in drug-activated SJS. Aim I will focus on establishing our novel drug-induced SJS mouse model. We will examine what immune changes occur as mice are dosed with specific SJS-causing drugs and then we will determine what role the specific Mrgprs play in these immune responses. In Aim II, we will examine the specific characteristics of Mrgpr-expressing dendritic cells. We will find what type of dendritic cell expresses the Mrgprs and the role they play in the immune system. We will then examine how the cells change upon exposure to SJS-causing drugs and how these changes cause a cytotoxic immune response and SJS phenotype. We will also confirm the pivotal role these Mrgpr-expressing cells play in the development of SJS by rescuing the phenotype in the Mrgpr KO mouse. In Aim III, we will continue examining the immune cascade triggered by the SJS drugs. Dendritic cells are known for their important role of presenting antigens to T cells, which activate and in turn cause an immune response. We will determine what T cells are being activated by the Mrgpr-expressing dendritic cells and what cellular effects and mediators are released by these activated T cells. We will establish the necessity of T cells in continuing the immune response that is initiated and maintained by activation of the Mrgpr-expressing dendritic cell. Data collected from this project will help reveal some of the cellular and molecular reasons behind certain drugs causing SJS; these new cellular and molecular targets could lead to new treatments and drug therapies for SJS and adverse drug reactions.